The present invention relates generally to the field of food processing and related food processing equipment and, more particularly, to the processing of semi-liquid food products, such as cheese. The cheese processing vat of the present invention and the method by which it operates preferably utilize a closed vessel in which generally cylindrical wall portions are oriented horizontally and enclosed with common end walls, the generally cylindrical wall sections having rotatable agitators which are mounted to sweep a common volume therebetween.
The use of totally enclosed processing vats, for the manufacture of many types of cheese and similar semi-liquid food products, is well known in the art. U.S. Pat. Nos. 3,858,855; 4,206,880; 4,938,424 and 5,178,060 are examples of prior art cheese making vats that are fully enclosed. The vats in each of these patents use agitator means, for both cutting the coagulum and stirring, that are vertically disposed within a vessel comprised of two partial cylinders. This design is very effective for smaller vessels but becomes less efficient with increases in size due to the need to create vertical movement within the vat contents without damagingly high peripheral agitator speeds. In the patents cited above, vertical movement is induced by means of hinged deflector plates which cause maximum turbulence when the agitator is rotated in the stirring direction while maintaining a highly streamlined configuration when the agitator is rotated in the cutting direction.
U.S. Pat. No. 4,108,058 discloses a fully enclosed cheese making vat in the form of a horizontal cylindrical vessel with a single horizontal shaft carrying the combined cutting and stirring paddles. The construction of the agitator paddle results in less damage to the food product and, in the case of cheese, results in minimum release of butterfat and cheese fines into the whey. A major disadvantage of this design is that the whole contents of the vat tend to rotate during cutting and in effect move away from the knife blades. This, together with a very high velocity gradient between the agitator blades near to the shaft and those on the periphery, makes it very difficult to achieve an evenly cut coagulum and can cause problems of product variability.
U.S. Pat. No. 4,989,504 describes a fully enclosed cheese making vat in the form of a plurality of horizontally arranged partial cylindrical sections each with a separate shaft-mounted agitator, such that the distance between agitator shafts is greater than the radius of the swept volume created by the rotation of the agitator. This arrangement is an improvement over the single shaft horizontal vat with regard to rotation of the vat contents during cutting, but the problem of velocity gradient between the agitator paddle near to the shaft and that at the periphery is only slightly reduced. The low agitator speeds that are typically used with this type of cheese vat give rise to very poor agitation adjacent to the agitator shaft and can also cause a significant problem with product congealing around the shaft. The vat construction described in the above U.S. Pat. No. 4,989,504 includes an agitator comprising a series of individual paddles attached to the shafts in staggered rows or in a row wrapped spirally around the shafts. A resultant problem with this arrangement is the extreme difficulty encountered when entering the vat for maintenance or hand cleaning due to the impossibility of parking the agitator paddles in a position that allows easy and safe movement around the inside of the vat.
In U.S. Pat. Nos. 5,513,559 and 5,606,907, the horizontal axis of each agitator is disposed centrally within each wall portion, the distance between the axes being smaller than the radius of the swept volume created by rotation of the agitator so that a common portion of the swept volume for each axis intersects and overlaps the swept volume of the adjacent axis. The overall agitation pattern is such that the contents of the vat will be induced to rotate in the same direction as the agitator with the contents subjected to significant cross-cutting action and other interactions in the zone where the agitator panels overlap.
Notwithstanding the advantages in the construction and operation of horizontal vats, particularly those utilizing two or more horizontally arranged partial cylindrical sections with agitators that overlap as they rotate through a common volume between the agitators, operational deficiencies remain. It is believed that one of the main operational problems in compound horizontal cheese vats results from the attempt to utilize the prior vertical vat technology without essential change. In other words, the success in the cheese industry of the double cylindrical wall vertical vat of U.S. Pat. No. 4,938,424 led subsequent innovators to attempt simply to turn that vat on its side and operate it in a horizontal mode. Thus, dual agitator horizontal vats of the types described above all utilize agitators which rotate in the same direction, such that one agitator rotates through a common chamber volume between agitators in one direction while the following agitator rotates through that space in the opposite direction. It was assumed that the processing improvements, both in stirring and cutting, provided by the double vertical vat constructions would automatically occur in similar vat constructions which were horizontally oriented.
Actual experience has not supported the foregoing assumption. Thus, in horizontal vats having two generally cylindrical outer wall portions oriented side-by-side and interconnected to provide a common center volume, driving the agitator panels in the same rotational direction (both for stirring and cutting), though providing benefits in increased capacity, uniformity of the cheese product, and somewhat reduced power requirements, deficiencies still remain. In particular, rotation of the agitator panels in the same direction, causing a bulk flow of the curds and whey generally around the walls of the vat, has been thought to be necessary for adequate temperature control and mixing. What typically occurs, however, is that the curds tend to pile up on the side of the vat where the agitator panel is moving upwardly. Curd motion tends to slow and stop and the curds start to undesirably mat together, unless additional mechanical energy is applied, such as increasing the speed of agitator rotation. Such increased velocity tends to over-agitate the cheese, causing solids loss of cheese particles into the whey. Such action is directly contrary to the desirable goal of keeping the cheese curds in suspension and evenly distributed at the lowest rpm. The agitator panel on the side of the vat providing the lift is overworked while the opposite agitator panel is under utilized. This often requires periodic reversal of the agitator to help move the curd to the other side. Such reverse movement is very inefficient because the cutting blades are used rather than the stirring edges and, in addition, the blade cause undesirable additional cutting of the curd and bleeding of whey therefrom.